Rhythmic motor activity in animals requires that different muscle groups be active with the appropriate phase relations. For example, in a swimming lamprey a traveling lateral wave of body curvature progresses from head to tail. Likewise, in the leech there is a posterior to anterior peristaltic wave of contraction of the heart tubes. In many animals the neural mechanism that controls these motor activities can be described as a chain of segmental oscillators because each segment can produce rhythmic activity in isolation. We have the studied the neuronal network that paces the heartbeat of the leech. This system is particularly amenable to a cellular analysis of the origin of oscillatory neural activity because the neural circuit consists of identified neurons that have been biophysically characterized. The aim of this proposal is to determine the dependence of the inherent cycle periods of segmental oscillators on intrinsic voltage-gated currents and to use computer simulations to help understand the intersegmental coordination of segmental oscillators.